Historically, proposals for extending the reference implementations do
not tend to go very far. Note that the last official release of
Markdown.pl was in 2004... With alternative implementations, your
chances are little bitter. Michel occasionally incorporates really
promising suggestions into PHP Markdown Extra. (I am guessing it would
need to be a lot more compelling.) We, of Python Markdown, tend to
remind people who ask for new features that we offer an extension
framework [1] that would allow you to change Python Markdown to do
things like you suggest in a few dozen lines of code.